PROJECT SUMMARY Hyperammonemia syndrome is a condition of previously unknown cause affecting approximately 4% of lung transplant recipients early after transplantation. Despite maximal supportive therapy directed at suppressing ammonia production and augmenting its excretion, most with this condition have historically progressed to cerebral edema and died. Using PCR, we detected Ureaplasma urealyticum, a bacterium which produces large amounts of urease, hypothetically leading to production of ammonia, in the blood and organs of a lung transplant recipient who died of hyperammonemia syndrome. We subsequently tested lower respiratory secretions, and/or lung tissue of 12 additional lung transplant recipients with hyperammonemia syndrome - seven tested positive for U. urealyticum and the other five tested positive for Ureaplasma parvum. Although seven diagnoses were made post-mortem, six were made while patients were alive, enabling treatment with Ureaplasma-directed therapy; all six survived. Importantly, we have found no evidence of pulmonary or systemic infection with these organisms in lung transplant recipients without hyperammonemia syndrome. There is also evidence supporting donor-transmission of Ureaplasma species to lung transplant recipients. Based on these clinical findings, we established immunosuppressed experimental murine models of U. urealyticum and U. parvum infection, which we used to show that infected mice developed hyperammonemia. We also performed antimicrobial susceptibility of Ureaplasma species, showing that it is not predictable (i.e., antibacterial resistance in Ureaplasma species is a challenge). Together, our finding of U. urealyticum or U. parvum in all lung transplant recipients with hyperammonemia syndrome tested to date, the ability of these organisms to generate ammonia from urea, and recapitulation of hyperammonemia in animal models, suggest that U. urealyticum and U. parvum are likely causes of hyperammonemia syndrome in lung transplant recipients. Notably, our preliminary studies suggest that appropriately directed antimicrobial therapy can reverse this syndrome. In our proposed studies, we will define mechanisms behind this unusual infection-associated metabolic syndrome, including determining whether U. parvum and U. urealyticum can produce sufficient ammonia to result in systemic hyperammonemia, and assessing how the immune system protects against Ureaplasma- associated hyperammonemia. Using experimental animals, we will delineate antibiotic prevention and treatment strategies for Ureaplasma-associated hyperammonemia, including cases associated with antibacterial resistance. Our research team has a unique set of skills and experience to carry out the proposed studies which will elucidate how a bacterium can cause a treatable metabolic syndrome in humans.